Alright, so I was messing around with parseInt to see how it handles values not yet initialized and I stumbled upon this gem. The below happens for any radix 24 or above.

parseInt(null, 24) === 23 // evaluates to true

I tested it in IE, Chrome and Firefox and they all alert true, so I'm thinking it must be in the specification somewhere. A quick Google search didn't give me any results so here I am, hoping someone can explain.

I remember listening to a Crockford speech where he was saying typeof null === "object" because of an oversight causing Object and Null to have a near identical type identifier in memory or something along those lines, but I can't find that video now.

6 Answers
6

It's converting null to the string "null" and trying to convert it. For radixes 0 through 23, there are no numerals it can convert, so it returns NaN. At 24, "n", the 14th letter, is added to the numeral system. At 31, "u", the 21st letter, is added and the entire string can be decoded. At 37 on there is no longer any valid numeral set that can be generated and NaN is returned.

@Ignacio. Actually, I was wrong. I didn't realize 37 was referring to a radix. Sorry about that.
– Mike SamuelJun 23 '11 at 20:15

3

@Robert, no I was confused and thought he was claiming something other than what he was claiming. It's the right answer. Apologies all around.
– Mike SamuelJun 23 '11 at 20:16

19

I still think that this answer could do with some references. Although it's entirely correct, it's really just one big assertion...
– Lightness Races in OrbitJun 23 '11 at 20:23

4

@Tomalak - check my answer for all the references. This answer is the correct (and first) one so I think it should remain the accepted one. Although it never hurts to explain what goes on under the hood ;)
– David TitarencoJun 24 '11 at 0:35

function parseInt converts its first
argument to a string, parses it, and
returns an integer or NaN. If not NaN,
the returned value will be the decimal
integer representation of the first
argument taken as a number in the
specified radix (base). For example, a
radix of 10 indicates to convert from
a decimal number, 8 octal, 16
hexadecimal, and so on. For radices
above 10, the letters of the alphabet
indicate numerals greater than 9. For
example, for hexadecimal numbers (base
16), A through F are used.

In the spec, 15.1.2.2/1 tells us that the conversion to string is performed using the built-in ToString, which (as per 9.8) yields "null" (not to be confused with toString, which would yield "[object Window]"!).

So, let's consider parseInt("null", 24).

Of course, this isn't a base-24 numeric string in entirety, but "n" is: it's decimal 23.

Now, parsing stops after the decimal 23 is pulled out, because "u"isn't found in the base-24 system:

If S contains any character that is
not a radix-R digit, then let Z be the
substring of S consisting of all
characters before the first such
character; otherwise, let Z be S. [15.1.2.2/11]

(And this is why parseInt(null, 23) (and lower radices) gives you NaN rather than 23: "n" is not in the base-23 system.)

This is very tragic behavior of parseInt (I was thinking why it was not design to though exception in these case). I would prefer to use NUMBER() instead when I can.
– Grijesh ChauhanMar 22 '14 at 13:57

Ignacio Vazquez-Abrams is correct, but lets see exactly how it works...

From 15.1.2.2 parseInt (string , radix):

When the parseInt function is called,
the following steps are taken:

Let inputString be ToString(string).

Let S be a newly created substring of inputString consisting of the first
character that is not a
StrWhiteSpaceChar and all characters
following that character. (In other
words, remove leading white space.)

Let sign be 1.

If S is not empty and the first character of S is a minus sign -, let
sign be −1.

If S is not empty and the first character of S is a plus sign + or a
minus sign -, then remove the first
character from S.

If stripPrefix is true, then a. If the length of S is at least 2 and the
first two characters of S are either
“0x” or “0X”, then remove the first
two characters from S and let R = 16.

If S contains any character that is not a radix-R digit, then let Z be the
substring of S consisting of all
characters before the first such
character; otherwise, let Z be S.

If Z is empty, return NaN.

Let mathInt be the mathematical integer value that is represented by Z
in radix-R notation, using the letters
A-Z and a-z for digits with values 10
through 35. (However, if R is 10 and Z
contains more than 20 significant
digits, every significant digit after
the 20th may be replaced by a 0 digit,
at the option of the implementation;
and if R is not 2, 4, 8, 10, 16, or
32, then mathInt may be an
implementation-dependent approximation
to the mathematical integer value that
is represented by Z in radix-R
notation.)

Let number be the Number value for mathInt.

Return sign × number.

NOTE parseInt may interpret only a
leading portion of string as an
integer value; it ignores any
characters that cannot be interpreted
as part of the notation of an integer,
and no indication is given that any
such characters were ignored.

There are two important parts here. I bolded both of them. So first of all, we have to find out what the toString representation of null is. We need to look at Table 13 — ToString Conversions in section 9.8.0 for that information:

Great, so now we know that doing toString(null) internally yields a 'null' string. Great, but how exactly does it handle digits (characters) that aren't valid within the radix provided?

We look above to 15.1.2.2 and we see the following remark:

If S contains any character that is
not a radix-R digit, then let Z be the
substring of S consisting of all
characters before the first such
character; otherwise, let Z be S.

That means that we handle all digits PRIOR to the specified radix and ignore everything else.

Basically, doing parseInt(null, 23) is the same thing as parseInt('null', 23). The u causes the two l's to be ignored (even though they ARE part of the radix 23). Therefore, we only can only parse n, making the entire statement synonymous to parseInt('n', 23). :)

I guess null gets converted to a string "null". So n is actually 23 in 'base24' (same in 'base25'+), u is invalid in 'base24' so the rest of the string null will be ignored. That's why it outputs 23 until u will become valid in 'base31'.